java中英文翻译
Characteristics of LANs
The main Characteristics of a LAN include its throughput,delays,wiring type and distances,security,and reliability.We discuss these characteristics below.We then compare the characteristics of major LANs.
Throughput
The throughput is the average total transmission rate when the LAN is heavily loaded by many nodes.
In a shared LAN,the throughput is a fraction of the rate of the transmitters.This fraction is called the efficiency of the MAC protocl.For insyance,a network that uses a MAC protocol with an efficiency oh 65 percent and 10-Mbps transmitters has a throughput of 6.5 Mbps.These values are typical for a shared 10-Mbps Ethernet.The throughput of a token ring network and that of FDDI are close to their thransmission rate. Latency
水性聚氨酯
图片轮显The latency is the time a packet takes froms its arrival at anetwork interface until it reaches the destination node.We explain in Section 4.8that this latency is almost aiways less than a fraction of a second and is often much smaller.
Wiring Type and Distance
Some LANs use twisted wire pairs,others use optical fibers,Older LANs and installations in special environments use coaxial cables.The maximum length of the connections depends on the LAN and on the techanology.Typical values are 100 m for twisted pairs and a few kilometers for multimode fibers.
Security
Anoptical fiber is more difficult to tap into than a twisted pair,which improves the “physical”security.However,eavesdroppers rarely need to tap directly into a link to gain access to information.A more common attack
is to use one of the computers attached on the LAN and configure it to read all the packet
s that travel on the network(this is called snooping).If sensitive pieces of information such as user passwords are transmitted in the clear(nonencrypted),then the eavesdropper obtains entry points into computers.As a rule,a switched LAN is more secure than a shared LAN because a computer on such a network sees only the packets intended for it.
Reliability
We depend increasingly on networks.Accordingly,reliability becomes more essential.As we mentioned in the opening remarks of this chapter,FDDA is designed to survive a link or node failure.An Ethernet keeps on working when some of its nodes or links fail.Atoken ring network can be wired with bypass electronics to have a similar reliability.
Network Devices
Text
There are many types of media for the network, and each one has advantages and disadvantages. One of the disadvantages of the type of cable that we primarily use (CAT
5 UTP) is cable length. The maximum length for UTP cable in a network is 100 meters (approximately 333 feet). If we need to extend our network beyond that limit, we must add a device to our network. This device is called a repeater.
The term repeater comes from the early days of visual communication, when a man situated on a hill would repeat the signal to the person on the hill to his right. It also comes from telegraph, telephone, microwave, and optical communications, all of which use repeaters to strengthen their signals over long distances, or else the signals will eventually fade or die out.
The purpose of a repeater is regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media. Watch
out for the Four Repeater Rule for 10Mbps Ethernet, also known as the 5-4-3 Rule, when extending LAN segments. This rule states that you can connect five network segments end-to-end using four repeaters but only three segments can have hosts (computers) on them.造船工艺 The term repeater traditionally me ant a single port “in” and a single port “out” device. But in common terminology today the term multiport repeater is often used as well. Repeaters are classified as Layer 1 devices in the OSI model, because they act only on the bit level and look at no other information. The symbol for repeaters is not standardized.
The purpose of a hub is to regenerate and retime network signals. This is done at the bit level to a large number of hosts (e.g.4,8, or even 24) using a process known as concentration. You will notice that this definition is very similar to the repeater’s, which is why a hub is also known as a multi-port repeater. The difference is the number of cables that connect to the device. Two reasons for using hubs are to create a central connection point for the wiring media, and increase the reliability of the network. The reliability of the network is increased by allowing any single cable to fail without disrupting the entire network. This differs from the bus topology where having one cable fail will disrupt the entire network. Hubs are considered Layer 1 devices because they only regenerate the signal and broadcast it out all of their ports (network connections).
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